Single-crystal silicon is used for most electronic applications. Exceptions exist, such as displays and some imagers, where amorphous silicon is applied to glass substrates in order to operate the display or imager pixel. In many applications, the display or imager is fabricated on top of the silicon electronics. For application to liquid crystal displays (LCDs), amorphous silicon has provided sufficient performance. For next generation display devices such as Organic Light Emitting Diodes (OLED), Active Matrix (AM) drive transistors made from amorphous silicon have proven problematic. Fundamentally, LCDs use voltage devices, and AM-OLEDs require current devices. Attempts to extend the conventional approach involve modifying the prior-art amorphous-silicon on glass. Amorphous-silicon is applied to the entire substrate panel, typically greater than two meters on a side, then is re-crystallized using large excimer lasers and scanning a line focus across the panel. The laser has to be pulsed so as to only melt the Si surface and not the glass. This technique results in the formation of poly-crystal silicon rather than single-crystal silicon.
The mobility of any type of amorphous or poly-crystalline transistor, including non-silicon and organic devices, is much smaller than the mobility of single-crystal silicon transistors. Electron mobility in amorphous silicon is ˜1 cm2/V·s compared to ˜100 cm2/V·s for poly-silicon, and ˜1500 cm2/V·s for high-quality single-crystal silicon. It is therefore advantageous to use single-crystal silicon in place of amorphous silicon in such devices. In a preferred embodiment of the present invention a plurality of planar single-crystal silicon regions on a non-silicon substrate at predetermined locations, for the purpose of electronic device fabrication is fabricated. For example, wafers of single crystal silicon are too costly for large displays and too small in size: Silicon wafers are typically 300 mm in diameter, compared to current LCD panels at more than 2 meters on a side. By comparison, approximately spherical particles, spheres or spheroidal particles of single-crystal silicon have been manufactured in large sizes less than or equal to 2 mm, which is large compared to individual pixel sizes. U.S. Pat. No. 4,637,855, incorporated herein by reference, entitled Process For Producing Crystalline Spherical Spheres, Filed Apr. 30, 1985 in the names of Witter et al., describes the manufacture of crystalline spheres.
In the past others have attempted to place diodes upon a curved surface of a silicon spheroid however this has proved to be challenging. In the prior art, attempts have been made to lithographically define structures on spherical surfaces, but this requires non-standard optics and has had limited success. Making electrical contacts to non-planar surfaces also requires non-standard techniques. The complexities involved in fabrication have prevented any real progress.
Curved surfaces of Si spheres have also been doped with an n-type dopant to form n-type Si surrounding a p-type Si region which comprises the majority of the surface of a sphere. An embodiment of this invention relates to the field of photovoltaic devices, in that the planar surface and region directly below can be doped for example with an n-type dopant and a region below with a p-type dopant so as to form a solar cell. A silicon sphere solar cell is described in a paper entitled Crystal Characterization of Spherical Silicon Solar Cell by X-ray Diffraction by Satoshi OMAE, Takashi MINEMOTO, Mikio MUROZONO, Hideyuki TAKAKURA and Yoshihiro HAMAKAWA, Japanese Journal of Applied Physics Vol. 45, No. 5A, 2006, pp. 3933-3937 #2006 The Japan Society of Applied Physics.
This invention however overcomes the limitations of the aforementioned prior art by conveniently utilizing the surface area and region about the planar surface on a planarized particle to fabricate electronic devices. A planar region having structures formed therein provides a convenient reliable way in which to provide electrical contacts to different parts of the device.
Another very important aspect of this invention is that it enables a technology that has a smaller carbon footprint by allowing circuits to be built that consume less power than similar circuitry which utilizes LCD technology.
In displays with previous generation LCD technology, white light is provided to the rear of the panel of the display, and each LCD pixel uses a filter to select Red (R), Green (G), or Blue (B) light. Filtering in this manner wastes ⅔ of the energy in the backlight. In addition the operation of the LCD pixel is dependent on the light being polarized, so further losses are incurred by the polarizer. In addition, part of each pixel is occupied by the amorphous silicon transistor, which blocks light coming through the panel.
The present invention enables production of large OLED panels, which are more efficient that LCD panels. OLED pixels emit at the desired color, R, G, or B only, so no energy is wasted creating other colors, which are then filtered out and which produce waste in the form of heat. In addition, the OLED emitters can be fabricated on top of the backplane electronics, so the emission area can be maximized without blocking light emitting areas of the pixel. By placing the backplane electronics out of the light path, the design can be optimized for speed and low power dissipation, as opposed to being compromised for light path requirements.